If one defines action based on where the rod bends, faster if it only bends near the tip and slower if it bends farther into the blank, then when you take an existing rod/blank and shorten it from either end the action will become slower. BUT, This is based on the CCS measurement process which gives objective data on power and action, AND it measures the blank/rod from the very ends. Where the grip is is not part of the process and does not affect the numbers. So that's a little quirk in the system, and how that affects what your feeling I won't hazard a guess.
ALSO, "fast action" does not mean a stiff tip. That is "power." A fact that most don't realize is that if you have two blanks of the same power, meaning they will deflect the same distance for the same load, the faster the action the softer/less stiff the tip will be. Think of it this way, if you have an extra extra extra fast action rod of quite high power it will flex mostly in the last few inches of the rod and will act like a broom stick with an ice rod on the end of it. It will still be very powerful but you will go through the tip very easily then come up hard on the broom stick section. Before everyone writing to tell me I'm full of bologna please take the time to try to understand the example above. AT THE SAME POWER, AS POWER IS TRADITIONALLY MEASURED, THE FASTER THE ACTION THE SOFTER THE TIP. IT HAS TO BE IN ORDER FOR THE RODS TO HAVE THE SAME POWER.
If you want a rod that will snap a spinnerbait off weeds you need a rod of the right "power" (stiffness) to do that. Action is not the issue. Power is.
Back to the broom stick. It is obviously very stiff, right? It is very high power. But what is its action? By the definitions commonly used its action is very very very slow. It does not bend only in the tip section; it bends over its whole length.
Regarding your pulsing the figure eight for muskies, you are talking about power and not action. The mod fast action rod is actually a lower power rod than the fast that you are comparing it to. You are right in perceiving that it is not following as directly, but it's not because of its action; it's because it is a lower power rod than the faster action one.
I hope this helps. If not, ask again and I'll have another go at it. You might want to check out the link below. The system was designed to evaluate fly rods, but it works for any rod. The interaction of power and action is very complex, and hard to understand, but it helps to keep strictly to the definitions of each. Many who discuss it do not and that causes confusion. For example, I saw on here the other day a writer state he had a "heavy action" rod. He surely meant heavy power, but using "action" when "power" is the right word is misleading and confusing. It's sort of like horsepower and torque. They are not the same and not interchangeable.
https://www.common-cents.info/